1. Field of the Invention
This invention relates to a shield tunneling machine suited for laying pipes according to a pipe propelling engineering method.
2. Description of the Prior Art
According to a pipe propelling engineering method, a shield tunneling machine is disposed at the forefront of a plurality of pipes to be propelled. The tunnel face is excavated by the operation of a cutter head provided on the machine and simultaneously the pipe and machine are subjected to thrust produced by a propelling jack adjacent to the rearmost pipe. Therefore, the pipe and machine are propelled into the ground excavated by the operation of the cutter head. The cutter head is disposed in front of a partition wall crossing a shield body and spaced from the partition wall.
During propulsion of the machine and pipe, substances excavated from the tunnel face, i.e., muck are introduced into a pressure chamber provided between the cutter head and the partition wall, i.e., a front area of the shield body through the cutter head to fill the front area. The muck filling the front area serves to apply a face earth pressure to the partition wall of the shield body and to apply a reaction of the partition wall to the tunnel face, thus resulting in maintaining the tunnel face stable by an equilibrium between the reaction and the face earth pressure without any collapse and bulging of the tunnel face.
One of the known shield tunneling machines of this type, as disclosed in Japanese Utility Model Public Disclosure (KOKAI) No. 60-178098, Patent Publication No. 61-102999 and Utility Model Public disclosure (KOKAI) No. 63-5097, includes a rotor for crushing large gravels contained in the muck and disposed in the front area such as to facilitate a discharge of the excavated muck. In this machine, the rotor is rotated eccentrically about the center axis of the shield body by a drive mechanism so that the gravels are pressed against the inner surface of the shield body for crushing. The crushed gravels are discharged to a rear area of the shield body together with the muck by a discharging machine without reducing the pressure in the front area.
Also, in this machine, bearing sections are filled with lubricant such as to make the rotation of the rotor and that of a shaft for supporting the rotor smooth and to protect the bearing sections, and seal means are disposed between the rotor and the partition wall. The bearing sections are defined from the front area by the seal means to prevent water and muck from entering the bearing sections
The prior seal means of this type comprises a mechanical seal provided with an annular recess provided in a portion of the partition wall opposed to the rotor and opened to the rotor to extend about the axis of the shield body, a ring disposed in the recess for movement in the direction of the axis of the shield body and a spring for pressing the ring toward the rotor.
However, according to the mechanical seal used in the prior shield tunneling machine, the ring is a tube having the uniform outer diameter and the diameter of a seal surface of the ring contacting the rotor is larger than that of the seal surface of the rotor contacting the ring. Therefore, along with the eccentric movement of the rotor, the seal surface of the ring is exposed to the front area. At this time, the ring may be urged into the recess against the spring force due to the pressure in the front area thereby degrading the sealing effect.
That is, the front area, particularly a space around a seal device is held at a pressure which is higher than that in the bearing section. However, according to the prior mechanical seal, when the seal surface of the ring is exposed to the front area, the pressure in the front area acts on that portion of the seal surface which is exposed to the front area. This pressure serves to draw back the ring into the recess against the spring force, since the prior mechanical seal is constructed to bring the seal surface into contact with the partition wall. Therefore, the ring is separated from the partition wall to degrade the seal effect.
On a front end of the rotor is mounted a cutter assembly provided with a plurality of cutter bits. Each cutter bit is disposed such that the cutting edge is located on the identical surface orthogonal to the rotary axis of the cutter assembly and directed radially outward from the center of the eccentric section. Also, the prior machine is provided with an internal gear fixed to the partition wall and an external gear fixed to the rotor such so as to forcibly rotate the rotor around the eccentric section of a crankshaft. Thus, the rotor and cutter assembly are turned (revolved) about the center axis of the shield body while being rotated (on their own axes) around the axis extending parallel to the center axis of the shield body. Accordingly, each cutter bit excavates the tunnel face when it is moved outward since the cutting edge is directed outward.
In such a prior art machine, however, since the cutter assembly revolves both around the shield body and on its own axis while the cutting edge of each cutter bit is directed outward, the shield body will change its orientation upward along with the excavation.
Namely, when the tunnel face is excavated by the cutter bits disposed below the rotary axis of the cutter assembly along with the turning and rotational movement of the cutter assembly, the cutter assembly is subjected to upward force. This upward force is applied to a front portion of the shield body. When the ground is soft, this force acts on the shield body so as to push up earth and sand above the shield body. As a result, a space is formed between the front lower surface of the shield body and the ground and the earth and sand around the shield body are introduced into this space to maintain the orientation of the shield body slightly upward. Thus, every time the excavation is done by the cutter bits disposed below the rotary axis of the cutter assembly, the orientation of the shield body will be changed gradually upward. Particularly, when soft ground including rocks is excavated, a large force acts on the shield body, so that the orientation of the shield body will be remarkably changed.
On the other hand, when the tunnel face is excavated by the cutter bits disposed above the rotary axis of the cutter assembly, the cutter assembly is subjected to downward force. Thus, the downward force also acts on the shield body. However, the lower surface of the shield body is only pressed against the earth and sand under the shield body due to the downward force. At this time, since a space is not formed between the lower surface of the shield body and the ground around the shield body, the orientation of the shield body will not be changed even if the ground to be excavated is soft.